The present invention relates to an improved electrically insulating silicone rubber sheet with good heat conductivity or, more particularly, to an electrically insulating silicone rubber sheet with good heat conductivity having adhesiveness exhibiting low contact resistance against thermal flow, which is suitable as a spacer sheet for holding heat-generating devices to be built in various kinds of electronic and electric instruments.
It is a common practice in the prior art that heat-generating electronic or electric devices and units, such as power transistors, thyristors, posistors, rectifiers, transformers and the like, are fixed to a heat-radiator or a metal-made chassis in order to effect good heat dissipation when they are built in an electronic or electric instrument since they are susceptible to the lowering in the performance or even breakage by the temperature elevation caused by the heat generated therein. It is not always desirable in such a mounting that the electronic or electric device or unit is contacted directly with the radiator from the standpoint of the performance or design of the instruments as well as safety so that the device or unit is fixed to the radiator or chassis usually with a spacer sheet of an electrically insulating material intervening therebetween.
The materials for such an insulating spacer sheet are diversified including various kinds of inorganic materials with good heat conductivity such as silica, alumina, boron nitride, magnesia, hydrated alumina, mica and the like and these materials are used as such in the form of a sheet or as dispersed in a finely divided form in the matrix of a cured silicone rubber sheet. These prior art spacer sheets are not quite satisfactory in their performance when used as such and, in addition, there is a problem of misplacing of the spacer sheet due to sliding when an electronic or electric device is mounted on a radiator with the spacer sheet therebetween.
When the insulating spacer sheet is made of an inorganic insulating material such as mica, it is known to coat the surface of the spacer sheet with an oily material so as to improve the contacting condition with the radiator. This method of coating with an oily material is not satisfactory even when the spacer sheet is made of a rubbery material due to the limitation in the adhesive contacting between the spacer sheet and a radiator with incapability to comply with very diversified surface conditions such as the depression caused by the devices or radiators, markings and the like and no sufficient heat conduction is obtained from the device to the radiator with large contacting resistance against heat flow.
Further, cured rubber sheets have no adhesive or sticking power to the surface of a metal and the like solid to prevent slippage so that an outer fastening means, such as screw fastening, is indispensable in order that the spacer sheet is fixed at the correct position between a device and a radiator. Such a fastening means is of course undesirable due to the limitation in the use of screws and the difficulties in the exact positioning even when the design permits the screw fastening. Moreover, the fastening force with the screws cannot be sufficiently large to obtain improvement in the thermal resistance since an excessively large fastening force may destroy the device or the spacer sheet per se.